Evidence for Detection of Raji-directed Immunoglobulin G Antibody in Sera from Patients with Systemic Lupus Erythematosus

at 37°C of IgG from the sera of 16 patients with systemic lupus erythematosus (SLE) to pronasedigested Raji cells was reduced much less consistently and extensively (9-100% reduction; mean reduction of 51%). In more detailed studies of two SLE sera, sucrose density gradient centrifugation showed that >50% of the IgG binding to undigested Raji cells sedimented in the 7S region. Pepsin digestion ofimmunoglobulin fractions from four SLE sera caused a reduction in SLE IgG binding to undigested Raji cells when detected with 125I anti-Fcy, but an increase when binding was detected with '25I-anti-Fab, suggesting that substantial SLE IgG can bind through F(ab')2 regions. Binding ofIgG from SLE sera was not directed at neoantigenic sites induced by pronase digestion because binding activity was adsorbed with undigested cells as readily as with digested cells. Moreover, sera from 10 SLE patients that had negative Raji assays contained no IgG that bound to pronase-digested Raji cells. We conclude that much of the IgG bound at 37°C to Raji cells from the sera of many patients with SLE does not represent immune complexes but is probably antibody directed toward sites on the Raji cell. Received for publication 11 February 1980 and in revised form 22 April 1980. INTRODUCTION The Burkitt lymphoma-derived human lymphoblastoid cell line, Raji, which bears receptors on its plasma membrane for immunoglobulin (Ig)'G and several complement components including C3-C3b, C3d, Clq, has been used for the past several years to detect small amounts of circulating immune complexes (IC) (1). The receptors for C3 have been thought to be primarily involved in IC binding (1), although recent evidence suggests that Clq receptors may play a larger role (2). Binding to the Fc receptors has been shown to be negligible (3). A clinical assay for circulating IC using this cell line has enjoyed wide use (1). A number of diseases have been shown to be associated with circulating IC, and clinical correlation of disease activity with Raji cell reactivity has been demonstrated in some cases (for review, cf. 4). The cell line has additionally been used to purify IC for purposes ofantigen identification (5). In the course of evaluating a number of antisera for usefulness in detecting IC on the surface of Raji cells, we discovered that goat IgG antibodies directed against human y, u, and a chains were all capable of detecting material adsorbed to Raji cells from sera from patients with systemic lupus erythematosus (SLE). Because antibodies to a number of lymphocyte determinants are known to circulate in the sera of these patients (6) it seemed reasonable to consider that such antibodies might be responsible for at least a part ofthe Raji cell activity. Such antilymphocyte antibodies, detected by cyto' Abbreviations used in this paper: AHG, aggregated human IgG; HSA, human serum albumin; Ig, immunoglobulin; NHS, normal human serum; PBS; phosphate-buffered saline; SLE, systemic lupus erythematosus. J. Clin. Invest. ( The American Society for Clinical Investigation, Inc. 0021-9738/80/08/0353/08 $1.00 353 Volume 66 August 1980 353-360 Downloaded from http://www.jci.org on June 24, 2017. https://doi.org/10.1172/JCI109863